gametogenesis & fertilisation

also development of the zygote

what path do primordial cells take?

through the gut→ genital ridge (become gametes)

what does the genital ridge turn into?

into gonads (testes or ovary)

describe the process of spermatogenesis

PGCs→ spermatogonia→spermatocytes→spermatids→spermatozoa

spermatogonia arrest in G1

spermatocytes enter meiosis

incomplete cytokinesis= syncytia=spermatids

spermatids differentiate into spermatozoa

describe how sperm differentiate?

-golgi apparatus develops into acrosomal cap

-flagellum develops

-cytoplasm extruded

-mitochondria coalesce near base of flagellum

arginine-rich proamines replace histones

nucleus condenses

cytoplasmic bridges lost

what 3 enzymes does the acrosome contain?

protein digestion- acrosin

carb digestion- B-N-acetylglucosaminidase

lipid digestion- phospholipase C

describe the stages of oogenesis?

PGCs→oogonia→primary oocytes→secondary oocytes→1 ovum + 2 polar bodies

oogonia meiosis and arrest i prophase 1= primary oocytes

as adult, upon ovulation: meiosis 1 completed, arrest in metaphase II=secondary oocyte

fertilisation allows meiosis II to complete= 1 ovum + 2 polar bodies

what are specialisations of egg cells?

-nutritive yolk proteins

-protein synthesis machinery

mRNA encoding proteins needed for early development

-morphogenic factors to direct early development

-protective chemicals e.g. against UV

-extracellular glycoprotein coat

what does the zona pellucida consist of? what is the sperm receptor?

ZP1, ZP2, ZP3 (this receives sperm, has O-linked polysaccharide allowing species specificity) (all glycoproteins)

what are the cortical granules made of? what are they developed from?

proteases, glycosidases, developed from golgi apparatus

what is the cumulus cell layer made of?

cumulus cells, matrix of hyaluronic acid (sperm has hyaluronidase activity)

describe how the reaction between GalT and ZP3 occurs?

-GalT recognises N-acetylglucosamine residues on ZP3, cross-linking

-causing clustering of GalT, triggers G-protein activation→voltage gated Ca2+ channels open

-inc intracellular Ca2+→ triggers exocytosis of acrosomal vesicle

-acrosomal enzymes lyse the ZP (including acetylglucosaminidase & acrosin-serine protease)

how do Izumo and Juno work together?

sperm Izumo recognises oocyte Juno→ recruit CD9→ plasma mbs fuse→ sperm enters

causes calcium release (this then diffuses across eventually into oscillations)- exocytosis of cortical granules (cortical reaction), completion of second meiosis

how are the cortical granules carried to the plasma mb?

actin polymerises to intermediate filaments→transport cortical granules

what do the enzymes in cortical granules do once released?

partially digest ZP2 & remove carb from ZP3→ ZP hardens blocking any other sperm→ no polyspermy

how does Ca2+ release cause the completion of meiosis?

activates a kinase that leads to proteolysis of cyclin

what does the sperm provide vs what does the egg provide?

sperm: a haploid genome, a centriole

egg: haploid genome, mitochondria & other organelles, mRNAs and proteins needed for early development

what does morphogenesis involve?

morphogenesis- creation of structure and form

differential proliferation

change in cell shape & shape

cell movement

cell fusion & death

what does gastrulation do?

moves the germ layers relative to one another

how would you describe zygotic cells vs embryonic cells?

totipotent vs pluripotent

what does cleavage of the zygote cell produces?

a cluster of blastomeres

what is formed 3 days after fertilisation (a cluster of blastomeres get more gap junctions)

morula→ undergoes compaction, E-cadherin restricted to only regions of intracellular contact

what type of cleavage does the morula undergo? what does this produce?

tangential, 1 polarised (have apical & dorsolateral surfaces) & 1 non polarised daughter cell (inner cell mass formed from this)

what do the inner and outer cells give rise to?

outer=trophectoderm

inner= inner cell mass

whole thing is blastocyst once fluid filled cavity develops

what does the trophectoderm develop into?

extra embryonic tissues

what does the ICM develop into?

the embryo proper

within a blastocyst, what stops implantation in the oviduct?

zona pellucida

how is the blastocoel developed?

tight junctions act as permeability barrier

Na+ is AT into blastocoel

water flows in by osmosis→hydrostatic pressure inflating blastocoel

what are some features of embryo hatching?

ZP is hatched out of

trophoblast derived from trophectoderm

hypoblast (from ICM)

Epipblast (from ICM)

blastocyst cavity

day 6

what are the 2 layers of the embryoblast? what are their fates?

epiblast- forms embryo proper

hypoblast- forms extra-embryonic structures

what type of cells are the epiblast?

columnar -adjacent to amniotic cavity

what type of cells are in the hypoblast?

small cuboidal- adjacent to blastocyst cavity

when does the primitive streak develop? where?

after 2 weeks, surface of epiblast (that form posterior)

what happens with epiblast cells during gastrulation?

migrate toward primitive streak, invaginate & displace hypoblast

through gastrulation the 3 germ layers formed: first invaginating cells=endoderm, next invaginators=mesoderm, remaining=ectoderm

what occurs during neurulation?

ectoderm folds along central axis→ neural tube (top fuses)

neural crest cells form near site fusion→ migrate away & epidermis fuses above the tube

for the neural tube development, what do paraxial mesoderm segments develop into?

somites-. develop the trunk, vertebrae, dermis and muscles

the endoderm is internalised and develops into what?

GI tract epithelium, stomach, liver, pancreas, epithelial lining of respiratory tract

what occurs at 3-8 weeks of the embryonic period?

organogenesis, from organ primordia

this is when most structural defects are induced

how would you describe epigenesis?

generation of a complex organism from few cell types in a basic pattern

what does somatic cell nuclear transfer demonstrate?

almost all somatic cells have a complete copy of the genome (so can regenerate a whole organism)

how does differentiation of cells occur?

partitioning of cytoplasmic determinants (e.g. P granules in c.elegans (those w p-granules=germ cells))

inductions via diffusible signals or cell surface interactions

what are the organisms that are great for experiments?

mice, zebrafish, fruit fly, chick, nematode worm, frog (Xenopus laevis)

what are the 2 layers of Xenopus eggs?

animal (pigmented), vegetal (non-pigmented)

what does VegT do?

binds DNA & activates expression of: transcription factors that mediate endoderm differentiation

nodal TGF(B) signalling factors that are secreted and induce responding cells→mesoderm

what is the evidence for VegT function?

normal development only occurs when:

-expressed in right cells at right time

-loss of function→ no endoderm and mesoderm is minimal

-gain of function→ ectopic expression of endoderm specific genes

-VegT found in cells that give rise to endoderm

what does Wnt signalling do?

causes B-catenin to accumulate in nucleus→ target gene expression (e.g. dorsal development, goosecoid expression (transcription factor))

it stops GSK-3(B) from destroy B-catenin

sperm entry triggers cortical rotation, what occurs after this?

translocating dorsal determinants (Wnt11 & GSK-3-binding-proteins)→ initiate Wnt signalling & make dorsoventral axis

how does B-catenin cause goosecoid expression?

B-catenin→ interacts with Tcf-3 → activates expression of siamois

siamois acts w proteins induced by nodal signalling→ activates expression goosecoid

what does high nodal (VegT) and B-catenin induce?

these gradients across the endoderm cause organisation

high VegT + B-catenin= spemann organiser

if VegT is low= ventral mesoderm

what does the organiser do?

secretes signals that pattern the mesoderm & neutralise the dorsal ectoderm

what is a muscle inducing transcription factor?

MyoD→ causes differentiation into skeletal muscle

can be seen as a master switch- initiate complex programme of differentiation

what type of molecule is Nodal?

TGF(B) signalling factor